The present invention relates to an aqueous surface treatment chemical for aluminum surfaces, and a method of treating aluminum surfaces. More particularly it relates to an aqueous surface treatment chemical for forming coatings with good corrosion resistance and highly durable hydrophilic nature and also with no or extremely reduced odor on aluminum fins of heat exchangers and a method of treating them.
Aluminum and its alloys are light and have good workability and heat conductance, so that they are widely used for fins of heat exchangers. Recently more and more air conditioners have been used not only for cooling but also for warming or dehumidification.
However, it is observed that moisture tends to be condensed and deposited as water droplets on the fin surfaces of air conditioners during cooling operations. If the fin surface is water repellent, this condensed water tends to be deposited in a hemispherical form on the fin surface or forms bridges between the fins, preventing smooth air flow, which in turn increases resistance of air flow, thereby decreasing heat exchange efficiency.
In addition, although aluminum and its alloys are essentially excellent in corrosion resistance, it is likely that the condensed water remaining on the aluminum fin surfaces for a long period of time forms an oxygen concentration cell, and that contaminants such as nitrogen oxides NO.sub.x and sulfur oxides in the air are absorbed and concentrated in the condensed water, so that a hydration reaction and a corrosion reaction are accelerated. Those produced by the corrosion are accumulated on the aluminum fin surfaces, which not only deteriorate heat exchange characteristics, but also are blown out of the air conditioners as white fine powders together with a hot air during the warming operation in the winter.
To solve these problems, various attempts have been made to form on fin surfaces coatings for improving their corrosion resistance and for increasing their hydrophilic nature.
Such coatings are generally classified into inorganic coatings and organic coatings.
Proposed as a method for forming inorganic coatings is a method comprising forming a corrosion-resistant coating, for instance, by a chromate treatment, and dipping it in an aqueous solution mainly composed of silicon oxide and sodium oxide to form a hydrophilic coating (Japanese Patent Laid-Open No. 59-13078) . However, despite the fact that such inorganic coating has good hydrophilic nature and corrosion resistance, it generates unpleasant odor peculiar to silica because it contains the silicates. And when it is used as pre-coated coil, the coating is cracked while cutting so that its corrosion resistance is reduced, and also severe tool wear takes place by cutting operation of such precoated aluminum plates.
Japanese Patent Laid-Open No. 58-80355 discloses a composition for forming a hydrophilic coating comprising (a) an alkali metal silicate having an SiO.sub.2 /M.sub.2 O molar ratio of 2-5, (b) hydrophilic silicic acid or silicate fine powders, and (c) a silane coupling agent or an ortho ester of a transition element in Group IV of the Periodic Table or a derivative thereof. The coating prepared from this composition has excellent hydrophilic nature but it disadvantageously generates unacceptable odor.
On the other hand, as a method of forming organic coating, a method of forming a hydrophilic coating on an aluminum surface by applying an aqueous solution comprising water-soluble or water-dispersible, high-molecular resin and silica fine powders to the aluminum surface, and then heat-curing the coating was proposed [for instance, Japanese Patent Laid-Open No. 55-99976]. However, since this coating contains silica fine powders, it suffers from cracking while cutting in the case of a precoated aluminum plate, leading to lower corrosion resistance. And tools are extremely worn during cutting operations and the coating has relatively poor hydrophilic nature. In addition, it causes air conditioners to release unacceptable if slight odor peculiar to silica.
Further, U.S. Pat. No. 4,503,907 discloses a heat exchanger coated with an aqueous coating composition comprising a plurality of spaced fins with narrow distance in parallel to form flow passages between fins and a plurality of heat transfer pipes passing through the fins, the both surfaces of the fins being coated with an aqueous coating composition comprising 100 parts by weight of a resin component for water paint in solid content, 5 to 95 parts by weight of a surface active agent and 5 to 65 parts by weight of synthetic silica and baked at a temperature of 120.degree. C. to 200.degree. C. for 10 to 40 minutes for curing to give a coating film of 3 to 20 .mu.m, whereby the fins are provided with hydrophilic surfaces having excellent corrosion resistance and surface hardness. However, this coating composition also contains silica which causes the generation of unacceptable odor.
In addition, U.S. Pat. No. 4,588,025 discloses a heat exchanger made of aluminum and comprising a tube and fins attached to the tube, the heat exchanger being characterized in that the fins are treated with a coating composition comprising an alkali silicate, a low-molecular weight, organic compound having carbonyl group, and a water-soluble, high-molecular weight organic compound, and are thereafter dried by heating, whereby a hydrophilic coating is formed over the surfaces of the fins. This hydrophilic coating too releases unacceptable odor because of the silicate.